The invention relates to a microswitch in micro-electromechanical systems. Components manufactured by means of specific methods and processes, such as the lithography method, are called micro-electromechanical or micromechanical systems (MEMS). They allow the realization of electrical or also mechanical functions on a smallest scale in the xcexcm range. Thus, for instance, microswitches for use in the radio part of mobile phones are known from Brown, Elliott R.; RF-MEMS Switches for Reconfigurable Integrated Circuits; IEEE Transaction on Microwave Theory and Techniques; Vol. 45; No. 11; November 98.
Micro-electromechanical components are formed of a plurality of thin layers of most different lateral structures lying on top of each other in a vertical direction and having most different material properties. According to the desired function the individual layers consist, for example, of conductive or insulating materials, or of materials with certain mechanical properties such as a spring constant. By corresponding processes also more complex three-dimensional structures can be produced. In a simplified fashion a microswitch can substantially be formed of three lateral layers, whereby the medium layer is again removed at the end of the manufacturing process. Thus, a microswitch consisting of a base element as the lowermost layer and a flexible switching element as the uppermost layer is formed. Both layers or, respectively, the elements of the microswitch formed thereby lie opposite each other at a defined distance, which is obtained by the remote layer disposed therebetween. Said distance largely corresponds to the deviation which has to be overcome by the flexible switching element so as to close a switching contact between the base element and the switching element. If the base element is, for example, a silicon substrate, an additional conductive layer will be disposed thereon as contact surface to which a voltage can be applied. The switching element may be made of a metallic material thereby forming itself the contact surface, to which a voltage can then be applied. Said material of the switching element is provided with a spring constant, and the switching element is at least partially connected with the base element. If a voltage difference is now applied between the contact surfaces, which together form the switching contact, the flexible switching element is deflected in the direction of the base element due to the so effected electrostatic attractive force, and the switching contact is closed. For achieving an attractive force as high as possible the dimensions of the contact surfaces lying opposite each other are as large as possible. For insulating purposes an additional oxide layer may be applied onto the contact surfaces. A direct voltage causing an electrostatic attractive force and an alternating voltage as signal to be switched can then simultaneously apply to the same contact surfaces. As was mentioned above, the flexible switching element is fixed at least on one point of its edge. In response to the type of fixing and the form of the flexible switching element the microswitches in micro-electromechanical systems are then commonly called cantilever switch, bridge switch or also membrane switch.
FIGS. 2a and 2b show the basic structure of a prior art microswitch configured as bridge switch in the opened and closed position. The flexible switching element S is fixed at two points of its edge on the base element G in such a manner that it has a defined distance toward the base element in the open position. Due to the spring constant of the selected material and the fixing the flexible switching element is provided with a reaction force counteracting the deflection of the switching elements. A contact surface KG is disposed on the base element G, which, together with the switching element S as additional contact surface, forms the switching contact. If a voltage is applied to both contact surfaces the switching element S is moved against the reaction force in the direction of the base element G due to the thereby effected electrostatic attractive force. If the voltage as applied exceeds a certain value, the switching contact S is closed. If the voltage is removed from the contact surfaces, the switching element S will go back to its original form due to the reaction force, so that the switching contact is opened. The drawback of such switches is that, due to atomic and molecular surface forces formed when the contacts are closed, the surfaces of the switching element and the contact surface of the base element may stick together. If the surface forces are stronger than the reaction force the switching contact can no longer open. For avoiding said agglutination it is proposed to additionally apply a dielectric layer on the contact. Furthermore, it may be conceivable to increase the reaction force of the switching contact by a corresponding form and material selection. This entails that a higher response force and, thus, a higher voltage is necessary for the closing so as to overcome said greater reaction force. However, exactly when such microswitches are to be integrated in MEMS components with a small voltage supply, this is not desirable and not applicable. Moreover, higher voltages and the so caused higher attractive force include the risk that the contact tends to agglutinate more easily when closing it, namely due to the so-called contact-shattering.
U.S. Pat. No. 6,143,997 discloses a microswitch operating at low voltages. The base element comprises a contact surface and a plurality of separate electrodes. Moreover, a plurality of layers having the function of clamps for the switching element are provided on the base element. The switching element is guided by said clamps and is freely movable in a deviation range defined by the clamps. Additional counter-electrodes are applied on the side of the clamps opposite the base element as additional layer. Due to the fact that the switching element is movable, i.e. not connected in a stationary manner, no mechanical reaction force is available for opening the switching contact, but, for the opening, a first voltage potential is rather applied to the counter-electrodes and a second voltage potential is applied to the switching element so as to cause an attractive force between the counter-electrodes and the switching element. For closing the switching contact a first voltage potential is applied to the electrodes of the base element and a second voltage potential is applied to the switching element. Furthermore, the gravitational force may additionally be utilized if the microswitch is in a suitable position. Due to the fact that there is no mechanical reaction force, only the attractive force defined by the voltage on the counter-electrodes acts to open the switching contact and counteracts the gravitational force given a corresponding position. Due to the smaller forces the risk that the contact surfaces stick together is smaller. It is, however, disadvantageous that such microswitches with the above-described structures in micro-electromechanical systems require additional and more complex layer structures, which render the manufacturing processes thereof more laborious and, thus, more expensive.
The present invention is therefore based on the object to provide a microswitch which counteracts the disadvantageous agglutination known from the prior art and guarantees an as easy as possible manufacturing process for the micro-electromechanical system.
In accordance therewith the invention is based on the idea to provide a microswitch consisting of a base, hereinafter called base element, and a movable element called switching element. The switching element is provided with a spring constant and is, at least with a part of its edge portion, connected with the base element in a fixed manner. Thus, when the movable switching element is deflected, a reaction force is generated, which is directed opposite to the deflection. Both, the base element and the switching element each comprise at least two electrodes, hereinafter called electrode and auxiliary electrode, whereby the electrode of the base element and the one of the switching element are disposed opposite each other at a defined distance. The auxiliary electrode in both, the base element and the switching element, is provided in a lateral direction at the same distance from the respective electrode. Moreover, the base element as well as the switching element are each provided with a contact surface, which together form the switching contact of the microswitch. The distance between the electrodes of the base element and of the switching element substantially defines the deviation required by the movable switching element for closing the switching contact. If, for opening the switching contact, a voltage with a first voltage potential is applied to the electrodes and a second voltage potential of the voltage to the auxiliary electrodes, the voltage difference formed thereby causes, in a lateral direction, an electric field between the electrode and the auxiliary electrode in the base element as well as in the switching element. In correspondence with the direction of the electric field an accumulation of negative and positive charge carriers occurs on the surface portions of the electrodes and the auxiliary electrodes, which are disposed directly opposite each other in a lateral direction. In an orthogonal direction thereto, i.e. in the direction of the deviation of the switching element, the electrodes having the same charge carriers are then each disposed opposite each other. In other words, for example, an accumulation of positive charge carriers on the surface portion of the electrode of the switching element is opposite an accumulation of positive charge carriers on the surface portion of the electrode of the base element. This analogously applies to the accumulation of negative charge carriers. Thus, repulsion forces are generated between the accumulations of the same surface charges on the electrodes with the same voltage potential. As said repulsion forces substantially act in the same direction as the reaction force of the switching element, they support the reaction force of the switching element precisely at the moment of opening. This means that precisely when the contact surfaces of the switching contact start to become released or separated, the repulsion forces as generated act initially in the direction of the reaction force. Due to the fact that, prior to the opening of the switching contact, the electrodes and, respectively, the auxiliary electrodes with the same voltage potential and, thus, surface charges with the same sign are disposed very closely to each other, the repulsion forces are at this moment particularly large because of the small distance. Due to the fact that the repulsion forces act in the direction of the reaction force, they support the same when the switching contact is opened and, thus, counteract a permanent agglutination of the switching contact. It is an advantage that additional mechanical measures such as the increase of the spring constant as described in the prior art are not required for the microswitch according to the invention. Moreover, the application of additional laborious structures like the clamps and counter-electrodes known from the prior art can be waived, so that additional laborious process steps can be avoided.
Additional advantageous embodiments and preferred developments of the switch according to the invention are described in the subordinate claims.